The present invention concerns a coaxial metallurgical connection between parts with greatly different thermal expansion coefficients.
The metallurgical joining together of coaxial parts such as, for instance, tubes, or bolts or rods with tubes, by means of welding or silver-soldering techniques, normally presents no difficulties. However, this is true only if the parts to be joined consist of the same materials, or at least of materials the thermal expansion coefficients of which are not very different. In those cases, however, where for technical or economic considerations tubes or other formed parts with greatly different thermal expansion coefficients are to be joined together in intimate contact, the problem exists that very great tension is produced in the zone of the joint if cyclic thermal stresses occur, and the joint therefore fractures and develops leaks. This danger exists particularly if numerous temperature cycles in the range of, for instance, 20.degree. and 1000.degree.C occur.
In a metallic connection of two tubes pushed over each other or a coaxial connection of a tube and a bolt, three-dimensional stress conditions occur in the zone of the joint in the case of temperature changes. The larger the difference between the thermal expansion coefficients of the two partners and the larger the difference between the instantaneous temperature and that temperature at which the stresses in the joint zone of the connection are a mimimum, the greater the stresses. These stresses occur in the axial and circumferential direction as shear stresses and in the radial direction as tensile or compression stresses. In heating up or cooling down, the direction of the stress is reversed. A joint zone is fractureprone particularly in the case that the force acting in the radial direction appears as a tension component. In those cases where the fracture elongation of the joint zone is smaller than the difference of the increase in diameter of the two partners of the connection, a crack occurs.
In all those cases where material partners with compatible thermal expansion coefficients are not expediently available, the problem therefore arises to find a design for the joint zone, in which the development of radial stresses is greatly reduced and which thereby becomes very largely insensitive to cyclic temperature stresses. Such a design should make possible here not only connections between metallic partners, but also between metallic and non-metallic, e.g. ceramic, partners.